The general BCs for a given family of Hamiltonians, i.e., all possible BCs satisfying the current-nullification requirement
can be derived using the formalism of Refs. [1]}, [2]},
which provides a universal “standardized” form, parameterized by unitary matrices.
Together with the Hamiltonian of the most general form,
these general BCs provide the continuum model of the most general form for a class-A system with a boundary.
If present, symmetries can also be naturally incorporated [3]}, [4]}, [5]} into the formalism of general BCs.
Symmetries typically provide additional constraints, which lead to subfamilies of general BCs that satisfy these symmetries.
These symmetry-constrained general BCs,
together with symmetry-constrained general Hamiltonian provide the continuum model of the most general form for the respective symmetry class.
